Developers for use in electrophotography are required to comprise toner particles having a small average particle size with a sharp particle size distribution, and have satisfactory coloring performance and uniform chargeability, in order to obtain high quality images and excellent developer durability when used in an electrophotographic system.
The image formation characteristics, such as resolution, sharpness, half-tone reproduction and photographic reproduction of images, can be improved by reducing the average particle size of the toner particles in a developer. Furthermore, when the average particle size of toner particles is reduced, the half-tone reproduction and the photographic reproduction, in particular, are improved. In addition, when the toner particles have a small average particle size, the particle size distribution of the toner particles does not change even when the developer is used for an extended period of time. Therefore, the obtained image quality is stabilized and the life of the developer can be prolonged.
Conventionally, toner particles are generally produced by mixing a resin, a dye or a pigment, and a charge control agent, then fusing and kneading the mixture, mechanically crushing the kneaded mixture after cooling to produce toner particles, and classifying the resulting toner particles. The toner particles obtained by the above-mentioned conventional method have the following shortcomings when the particle size of the toner particles is reduced:
(1) As the particle size of the toner particles is reduced, the chargeability of the toner particles is decreased because of the non-uniform dispersion of the charge control agent in the toner particles, and the toner particles tend to be deposited on the background area of a copy paper, and to be scattered.
(2) The toner particle obtained by the above-mentioned crushing process has a very rough surface and many protrusions on the surface, so that the components of the toner particle are easily detached from the surface thereof As a result, a so-called toner filming phenomenon occurs, in which the toner particles are deposited in the form of a film on the surfaces of the carrier particles, a triboelectric charging member and a photoconductor employed in a photographic copying system
(3) The coloring performance of the toner particles is decreased when the particle size of the toner particles is reduced.
(4) It becomes difficult to remove the small-particle-size toner particles from the surface of the photoconductor when the photoconductor is cleaned.
(5) The productivity and yield of the small-particle-size toner particles are significantly low. In particular, when toner particles with a sharp particle size distribution are obtained, the productivity and yield thereof are considerably decreased and the cost of such toner particles is high. Also, there is a limitation on the reduction of the particle size distribution even when the classification is repeated.
Accordingly, many proposals have been made concerning the production of toner particles having small particle sizes and a narrow particle size distribution which are capable of yielding high image quality and which have high durability.
For example, a method of producing core resin particles including a pigment and a charge control agent therein by a suspension polymerization method is proposed as disclosed in Japanese Patent Publications 51-14895 and 47-51830. However, with this method, it is difficult to remove a polymer-dispersion stabilizing agent and a surface active agent remaining on the surface of toner particles. These agents often cause the deterioration of the chargeability of the toner particles, and the properties of the toner particles obtained by this method are easily changed depending upon the ambient conditions.
Moreover, it is difficult to stably produce toner particles having small particle sizes with a narrow particle size distribution.
Methods of producing toner particles having small particle sizes and a narrow particle size distribution with a so-called core-shell structure are proposed as disclosed in, for example, Japanese Laid-Open Patent Applications 58-106554, 61-18965, and 61-275766. According to these methods, a coloring agent and materials capable of imparting properties necessary for a toner for use in electrophotography are deposited and coated on the surface of particles with a narrow particle size distribution. The toner particles obtained by these methods, however, have very poor electric characteristics and durability because of the presence of the above coloring agent and materials on the surface of the toner particles.
Furthermore, methods of producing toner particles by dyeing resin particles by immersing the particles in a solution of a dye are disclosed in, for example, Japanese Laid-Open Patent Applications 50-46333, 1-103631, 56-154738, 61-228458, 63-106667, and 64-90454.
The methods disclosed in these Japanese Laid-Open Patent Applications appear suitable for producing toner particles with a narrow particle size distribution because of the small number of production steps. However, those methods have not yet been studied to the extent that it can be confirmed that each of the toner particles obtained is uniformly dyed to the inside thereof. Moreover the descriptions in these Japanese Laid-Open Patent Applications are insufficient for actual preparation of the toner particles.
Japanese Laid-Open Patent Application 61-228458 discloses a method of producing toner particles by dyeing resin particles prepared by dispersion polymerization, with a dispersion stabilizer permanently deposited on the surface of the toner particles. The toner particles produced by this method, however, have the shortcoming that the triboelectric chargeability thereof is unstable because the polarity of the toner particles is controlled by the dispersion stabilizing agent deposited on the surface of the toner particles.
Toner particles free from the above-mentioned shortcomings are proposed in U.S. patent application Ser. No. 596474 filed Oct. 12, 1990 by Suguro et al. These toner particles are prepared by dyeing polymer particles obtained by polymerization in a hydrophilic organic solvent and have a narrow particle size distribution. A developer comprising the above toner particles is capable of providing images with high image density and high resolution, free from toner deposition on the background of the images, and can be used for an extended period of time with high reliability, without causing significant changes in the image quality obtained and the properties of the developer
However, in the case of a developer which is prepared by mixing such toner particles with resin-coated carrier particles which are widely used for prolonging the life of developers and preventing the deposition of carrier particles on solid image areas, the developer has the defect that non-developed portions tend to be produced at the rear edge of a half-tone portion of the obtained image.